1. Field of the Invention
The present invention relates to a low thermally conductive thermal barrier coating system and to a method of manufacturing the same and, more particularly, relates to a low thermally conductive thermal barrier coating system which can effectively reduce the temperature of a metal substrate because its ceramics thermal barrier layer has low thermal conductivity, and which can exhibit excellent heat resistance and excellent durability for a long period when applied to high-temperature components such as gas turbine parts and jet engine parts, and relates to a method of manufacturing the same.
2. Description of the Related Art
In view of prevention of global warming caused by carbon dioxide gas emitted during combustion of a fossil fuels and improvement in economic efficiency by means of resource saving, further improvement in thermal efficiency is required for prime movers such as gas turbines and jet engines, and thus intensive research has been performed. In gas turbine power generating installations, it has been known that generating efficiency is further improved by the burner outlet gas temperature being raised by increasing the operating temperature. To enable operation at high temperature, metallic materials having high heat resistance has been continuously researched.
To improve durability (reliability) of the heat resisting metallic material, research has been conducted to improve heat resistance of the metallic material itself. For example, heat resistance superalloys made of a Ni-based alloy, a Co-based alloy, a Fe-based alloy or the like have intensively been studied as a structural material for high-temperature parts and various heat resistant superalloys have been put into pratical use.
However, high-temperature components made only of a superalloy of the prior art do not have sufficiently high melting point and are likely to cause softening and decrease in strength due to recrystallization in a high-temperature range, thus causing such fatal restriction that the members cannot be used at a high-temperature of 1000° C. or more.
As a remedy for the restriction, a thermal barrier coating (TBC: Thermal Barrier Coating) technique has been developed and put into practical use in part. The thermal barrier coating technique has a function of preventing temperature rise of a metal substrate by forming an oxide type ceramic layer having low thermal conductivity on the surface of the metal substrate to block heat.
FIG. 2 is a sectional view showing an example of configuration of a heat resisting structural member formed with the thermal barrier coating (TBC) of the prior art. The thermal barrier coating system shown in FIG. 2 has a three-layered structure composed generally of a metal substrate 1 made of a superalloy containing Ni, Co or Fe as a main component, a metal bonding layer 2 made of an MCrAlY (wherein that M is at least one kind of Ni, Co, Fe) alloy, platinum aluminide or the like having excellent corrosion resistance and excellent oxidation resistance formed on one surface of the metal substrate 1, and a ceramics thermal barrier layer (thermal barrier coating layer) 3 containing ceramics such as Y2O3 stabilized ZrO2 as a main component. Ceramics layers are generally provided by a plasma spraying method.
Consequently, an operation and effect of suppressing temperature rise of the metal substrate 1 by a thermal barrier effect of the ceramics thermal barrier layer 3 can be obtained. The metal bonding layer 2 also exerts an effect of reducing thermal stress generated between the metal substrate 1 and the ceramics thermal barrier layer 3, preventing corrosion of the metal substrate 1 and suppressing oxidation.
However, the high-temperature component (heat resisting structural member) formed with a thermal barrier coating layer of the prior art had problems in that it was likely to cause cracking and spalling of the ceramics thermal barrier layer and was inferior in durability and reliability. It is considered that cracking and spalling of the ceramics thermal barrier layer are caused by differences in thermal expansion coefficient between the ceramic thermal barrier layer and the metal bonding layer, sintering and transformation of the ceramics thermal barrier layer, and volume expansion due to oxidation of the metal bonding layer.
Once cracking and spalling occur in the ceramics thermal barrier layer, thermal barrier properties drastically deteriorate, causing rapid temperature rise of the metal substrate. In the worst case, the metal substrate may be melted or broken. Such a risk should be avoided for operation of the equipment.
As a new method of forming a ceramics thermal barrier layer to be replaced by the plasma spraying method of the prior art, an electron beam physical vapor deposition (EB-PVD) method has attracted special interest recently. Since the ceramics thermal barrier layer synthesized by the EB-PVD method has a columnar structure including many longitudinal cracks and thermal stress can be reduced by deformation of the longitudinal crack portion, thermal shock resistance is noticeably improved.
However, the ceramics thermal barrier layer synthesized by the EB-PVD method had a problem in that it is inferior in thermal barrier effect to the conventional ceramics thermal barrier layer synthesized by the plasma spraying method because of its high thermal conductivity. In the case of the low thermal barrier effect, the temperature of the metal substrate increases and oxidation is accelerated, and thus spalling of the coating film is likely to occur. It is, therefore, considered that the EB-PVD film having low thermal conductivity leads to an improvement in properties of the thermal barrier coating.
As a finding with respect to providing the ceramics thermal barrier layer with low thermal conductivity, it is employed to provide a plurality of layers in the columnar structure (see, for example, Patent Document 1: Japanese Patent Application, First Publication No. Hei 11-256304 (page 1, FIGS. 1 to 6)). It is also reported to form a zig-zag pattern by controlling orientation of a columnar structure (see, for example, Patent Document 2: U.S. Pat. No. 6,455,173(B1) (page 1, FIGS. 2 to 3)). There is also a technical report that a low thermally conductive substance such as Gd2Zr2O7 to be replaced by the partially stabilized ZrO2 of the prior art is used as a constituent material of the ceramics thermal barrier layer (see, for example, Patent Document 3: U.S. Pat. No. 6,258,467 (page 1, FIG. 2)).
However, the use of a special deposition apparatus and a special technique is indispensable to control the texture of the ceramics thermal barrier layer so as to provide plural layers in the columnar structure, as disclosed in Patent Document 1, or to form a zig-zag pattern by controlling orientation of the columnar structure, as disclosed in Patent Document 2, and also there was drawbacks such as high equipment cost, high manufacturing cost and high operating cost of the equipment. Therefore, the above techniques are not suited for practical use. Even if low thermal conductivity is achieved by the technique, sintering occurs at high temperature and nanopores or gaps, which are effective to achieve low thermal conductivity, disappear, resulting in high thermal conductivity.
As disclosed in Patent Document 3, when the low thermally conductive substance such as Gd2Zr2O7 to be replaced by the stabilized ZrO2 of the prior art is used as the constituent material of the ceramics thermal barrier layer, the low thermally conductive substance is inferior in mechanical properties such as erosion resistance and effective means for overcoming the problem and achieving low thermal conductivity has never been established. In the case in which the thermal barrier coating system is applied to high-temperature components such as gas turbine parts and jet engine parts, when spalling of the ceramics thermal barrier layer occurs, thermal barrier properties drastically deteriorate, causing rapid temperature rise of the metal substrate, and thus the member is melted or broken, resulting in a serious obstacle during operation of the equipment.
In actual gas turbine parts, for example, turbine blade, cooling gas holes having a size of about φ 1 mm is provided on the surface of a blade and a cooling gas is ejected from the inside of the blade, thereby suppressing the temperature from rising. In the case in which a thermal barrier layer is formed by thermal spraying, since the cooling holes are covered with the thermal spraying material, it is necessary to do further steps such as forming the cooling holes again after coating. In the case in which an EB-PVD process is applied to a conventional material to form a thermal barrier layer, although the cooling holes are not completely covered, the coating material is deposited around the opening portion, thereby causing problems in that the amount of the cooling gas decreases and satisfactory cooling properties cannot be obtained.